The single biggest atrocity in the long battle for control of Ivory Coast has emerged after aid workers discovered the bodies of up to 1,000 people in the town of Duekoue.

Charity workers who reached Duekoue said it appeared the killings had taken place in a single day, shortly after the town fell to troops loyal to Alassane Ouattara, the man internationally-recognised as having won last year’s presidential election.

The apparent massacre came despite the presence of United Nations troops and – if confirmed – will cast a shadow over Mr Outtara’s assumption of the Ivory Coast’s presidency after a four-month battle to oust Lawrence Gbagbo, the former president who lost the November election but refused to step down.

William Hague, the Foreign Secretary, said he was “gravely concerned” by the violence and loss of life in Ivory Coast and added: “I am determined that all alleged human rights abuses… must be investigated and those responsible held to account.

The International Committee for the Red Cross said its staff discovered more than 800 bodies of people who were clearly local civilians. They were mainly men who had been shot and left where they fell, the organisation said, either alone or in small groups dotted around the town, which lies at the heart of Ivory Coast’s economically crucial cocoa producing region.